1. Field of the Invention
The present invention relates generally to an optical device, and in particular, to an optical coupling device for coupling light into an optical waveguide device.
2. Description of the Related Art
The recent technological trend is toward fabrication of a low-cost optical module for an optical communication system. Optical coupling is very important to system stability, mass production, and cost reduction. For future-generation optical networks, small, easily aligned optical devices must be developed. One of the current optical devices having the most effective optical coupling characteristics is a lensed fiber.
Effective optical coupling between a laser diode (LD) and an optical fiber is important in an optical communication system. Compared to “butt coupling” or “bulk optics-based coupling”, coupling using a lensed fiber offers many benefits. It provides high coupling efficiency, reaching almost 100% in particular cases. Due to the small area of the lensed fiber, it is possible to fabricate a small-size optical module or a coupled LD array. Also, without using an additional device for coupling, thermomechanical stability of lensed fibers is also high.
There are many approaches to fabricating a lensed fiber. An optical fiber having a micro-lens, a polished end, or a laser micro-machined end each exhibit a coupling efficiency nearing 100%. In this optical fiber, light emitted from an LD is coupled directly to a fiber core. Hence, a working distance is very small, merely the diameter of the core. This limitation causes many problems when integrating the lensed fiber into an optical module . The working distance can be increased to about 20 μm using a tapered hemispherical-end fiber. A graded index multimode fiber end can increase the working distance up to 45 μm, though having loss up to a 4 dB. A silica fiber having an aspherical end face can increase the working distance remarkably, up to 153 μm.
In the above cases, axial and lateral misalignment must be relatively small, and the possibility of misalignment has a great impact on mass production and reproducibility of optical modules. While a lensed fiber using an expanded-core fiber and a hemispherically-ended coreless fiber has a long working distance, it suffers a low coupling efficiency of about 4 dB and a small lateral misalignment tolerance of about 1.5 μm. Another coupling device using a pair of graded index-fibers having hemispherical ends has a relatively long working distance of about 50 μm and a great coupling efficiency of 1.5 dB. Yet, its misalignment tolerance is far smaller than that of other lensed fibers and thus only active alignment is allowed. All the above lensed fibers are very complex to fabricate and have low reproducibility.
FIG. 1 illustrates the structure of a conventional lensed fiber and FIGS. 2 to 5 depict the characteristics of the lensed fiber. Referring to FIG. 1, to describe the optical coupling of a conventional lensed fiber 110, an LD 140 is also illustrated. The LD is aligned with the lensed fiber 110 with respect to an optical axis 150. The lensed fiber 110 is divided into a single mode fiber 120 and a hemispherically ended coreless tip 130 connected to the single mode fiber 120. As illustrated in FIGS. 2, 3 and 4, it is impossible for the tip 130 to transfer all incident optical power to the single mode fiber 120 because of severe limitations on incident height h and incident angle Φ, each shown in FIG. 1. The more influential factor is incident height h.
FIG. 2 illustrates incident angle-incident height curves 161, 162 and 163 when a working distance D is 130, 150 and 170 μm, respectively in the case where the end of the tip 130 has a curvature radius R of 75 μm and a length L of 1000 μm.
FIG. 3 illustrates incident angle-incident height curves 171, 172 and 173 when R is 130, 150 and 170 μm, respectively in the case where D=150 μm and L=1000 μm.
FIG. 4 illustrates incident angle-incident height curves 181, 182 and 183 when L is 800, 1000 and 1200 μm, respectively in the case where R=75 μm and D=150 μm.
To enhance coupling capability, the single mode fiber 120 has a thermally expanded core 125 at its end. This kind of single mode fiber 120 is characterized by its normalized frequency being maintained during fabrication. Therefore, the product of a maximum incident height and a maximum incident angle is kept as a constant during thermal expansion of the end of the core 125. Another parameter to consider in the single mode fiber 120, is that as a modal field diameter increases, the diameter of the end of the core 125 increases and a relative refraction index difference decreases.
FIG. 5 illustrates an incident angle-incident height curve 191 of the thermally expanded core 125 and an incident angle-incident height curve 192 of a non-thermally expanded core (not shown). Referring to FIG. 5, a larger amount of optical power can be coupled to the expanded core 125 under the above-described conditions. The end of the thermally expanded core 125 is similar to a tapered optical waveguide in many respects. In one respect, the relative refractive index difference is drastically decreased during thermal expansion. For example, if the diameter of the section of the core end, as taken perpendicularly to a Z axis, increases from 4 to 16.8 μm, the relative refractive index difference falls from 0.356 to 0.02%.
In summary, the conventional lensed fibers offer many benefits including high coupling efficiency, small size, and high stability. However, they have the shortcomings of complex fabrication and low reproducibility. Moreover, they have relatively short working distances and small lateral misalignment tolerances. What is worse, the conventional lensed fibers require precise, expensive tools such as V grooves to achieve high coupling efficiency.